1. Field of the Invention
This invention relates generally to highly efficient, high power, TEM.sub.00 lasers that have low sensitivity to misalignment with high beam pointing stability, and more particularly to diode pumped lasers that are nearly confocal, or between confocal and concentric, and that utilize crystals with strong thermal focusing, and to lasers of this type that are Q-switched.
2. Description of Related Art
There are many laser applications where insensitivity to misalignment and beam pointing stability are critical. These characteristics are desirable in order that the laser power and mode do not degrade upon vibration, shock or thermal cycling. There are also applications where an insensitivity of a laser's output to changes in its thermal lens are highly desirable; for example, a Q-switched laser that is used in an application requiring dynamic variations in repetition rate.
Confocal or nearly confocal resonators are suitable candidates when misalignment sensitivity and beam pointing stability are important considerations. With confocal resonators, the mode diameter throughout the resonator does not vary by more than about the square root of two. Another description is that the laser resonator is about 2 times as long as the Rayleigh range of the intracavity mode. The mode profile in a "concentric resonator" is somewhat different; it is analogous to the mode profile that would be found in a sphere. In fact, one type of perfectly concentric resonator is a simple two mirror resonator where the surfaces of the concave end mirrors define the surface of a sphere. In this type of concentric resonator, the mode diameter can be very small at one part of the resonator, but very large in another part of the resonator. In this case, the laser resonator is much longer than two Rayleigh ranges of the intracavity mode. Of course, there are analogous resonators that are in between these descriptions, or between the confocal and concentric descriptions. For example, a resonator might be approximately four times longer than the Rayleigh range of an intracavity waist of the eigenmode of the resonator.
A greater understanding of confocal or nearly confocal resonators can be ascertained with reference to FIG. 1(a), a stability diagram with two axes, g.sub.1 and g.sub.2. The classic values of the stability parameters g.sub.1 and g.sub.2 are defined as: EQU g.sub.1 =1-L/R.sub.1 EQU g.sub.2 =1-L/R.sub.2
where L is the length of the resonator and R.sub.1, R.sub.2 are the respective radii of curvature of mirrors M.sub.1 and M.sub.2 of the resonator, shown in FIG. 1(b). A two mirror laser resonator is stable if 0&lt;(g.sub.1)(g.sub.2)&lt;1. The TEM.sub.00 mode size of a resonator can be represented in terms of g.sub.1 and g.sub.2, and the stability parameters can also be used to judge misalignment sensitivity and other practical resonator characteristics. This classic discussion can be found in W. Koechner, Solid State Laser Engineering, 3rd edition, Springer-Verlag, N.Y., p. 204-205 (1992).
It should be noted that this stability analysis can be extended to a resonator with an intracavity lens or lenses, shown in FIG. 1 (c). The intracavity lens can be a conventional lens or a thermal lens. A thermal lens can be generated in a laser crystal by diode pump light, or by lamp pump light. In this case, the g parameters become: EQU g.sub.1 =1-(L.sub.2 /f)-(L.sub.0 /R.sub.1) EQU g.sub.2 =1-(L.sub.1 /f)-(L.sub.0 /R.sub.2)
where L.sub.0 =L.sub.1 +L.sub.2 -(L.sub.1 L.sub.2 f), and L=L.sub.1 +L.sub.2, as shown in FIG. 1(c), R.sub.1 and R.sub.2 are defined as in the case of no lens, and with f as the focal length of the intracavity lens which can be a thermal lens. This background is also presented in W. Koechner, Solid State Laser Engineering, 3rd edition, Springer-Verlag, N.Y. p. 204-205 (1992).
It should therefore be noted that the traditional two mirror stability analysis, with stability parameters g.sub.1 and g.sub.2, is still useful when more complicated, multiple mirror and lens resonators are being considered.
With reference now to FIG. 1 (a), in the upper right quadrant, a hyperbola 10 defines a region 12, bounded between hyperbola 10 and the g axes. For a resonator with values of g.sub.1 and g.sub.2 within region 12, a gaussian mode can exist. Hyperbola 10 has the characteristic that (g.sub.1)(g.sub.2)=1. Region 12 represents a stable regime; a gaussian mode can exist between the two mirrors defining the resonator. In the lower quadrant 14, another stable regime is found; gaussian modes can also exist for these g values. The ideal confocal resonator corresponds to point 16, the intersection of the g.sub.1 and g.sub.2 axis, where g.sub.1 =g.sub.2 =0. A resonator can be called nearly confocal if g.sub.1 and g.sub.2 are not too large.
In FIG. 2, an ideal confocal resonator 18 is defined by two opposing mirrors, 20 and 22, each having a radius of curvature R.sub.1 and R.sub.2, respectively. Mirrors 20 and 22 are separated by a distance L. For an ideal confocal resonator 18, R.sub.1 =R.sub.2 =L. If a lens is used at the center of the cavity and flat mirrors are used, a confocal resonator arises when f=L/2. A combination of lenses and curved mirrors can also produce a confocal resonator.
Referring again to FIG. 1 (a), a plane parallel resonator corresponds to points indicated at 24, where g.sub.1 =g.sub.2 =1. The values of g.sub.1 and g.sub.2 place this resonator right on the edge of the stability diagram, and for applications requiring insensitivity to misalignment and pointing stability, the plane parallel resonator is typically not the best choice.
A large radius resonator exists when R.sub.1 and R.sub.2 are much larger than L, and corresponds to points 26 in FIG. 1(a). Large radius resonators are useful for diode pumping. In one embodiment of a large radius resonator, a Nd:YLF crystal is used and the mode size of the TEM.sub.00 is large, which facilitates conventional mode-matching. This resonator is, however, sensitive to misalignment, and may have reduced pointing stability. As an example, a large radius resonator of length L with R=10L, is five times more sensitive to misalignment than the confocal resonator of length L. This is also shown by the Koechner reference.
A concentric or spherical resonator, where R.sub.1 =R.sub.2 =L/2, is represented at 24. In this case, g.sub.1 =g.sub.2 =-1. It has the properties that the mode can be very large at one point of the resonator, and very small at the other. This is mainly because the mirrors are on the surface of a sphere. The TEM.sub.00 modes of such a resonator are very small at the center of the resonator, but very large at the ends. An equivalent resonator can be constructed using a lens or lenses and flat mirrors, or with a combination of lenses and curved mirrors.
A half symmetric resonator 30 is illustrated in FIG. 3. It is defined by a curved mirror 32 and a flat mirror 34, separated by a distance L. In this instance, R.sub.1 is the radius of curvature of mirror 32. Because mirror 34 is flat, it has an infinite radius of curvature. A half symmetric resonator can be made to have equivalent properties with that of a confocal resonator, if g.sub.2 =1 and g.sub.1 .about.1/2. In FIG. 1(a), the half symmetric confocal resonator corresponds to point 36 on the stability diagram. A half symmetric resonator can also be a large radius resonator, where one mirror is flat, and the other has a curvature R.sub.1 &gt;&gt;L. Again, an equivalent resonator can be constructed using a lens or lenses and flat mirrors, or with a combination of lenses and curved mirrors.
Because a confocal resonator is relatively insensitive to misalignment, a mirror can be tilted a certain amount and the laser output power does not drop as rapidly as with other types of lasers. The mode in the confocal resonator does not change or move very much as mirrors are tilted. Because the confocal's pointing stability and its misalignment sensitivity are very good, for example as a function of environmental changes, this type of resonator has some useful properties.
A major disadvantage of a confocal resonator is that it has the smallest average TEM.sub.00 mode diameter of any resonator of any length L. In essence, the TEM.sub.00 mode volume of the confocal resonator of length L is smaller than that of other resonators of length L. This makes conventional mode matching difficult.
This is taught in A. E. Siegman, Lasers, University Science Book, Mill Valley, Calif., p. 750-759, 1986. Because of its small average size, the TEM.sub.00 mode in a confocal resonator is not very effective in extracting power from a large diameter gain medium. Additionally, because of the small average size of the TEM.sub.00 mode, a confocal resonator is very likely to oscillate in a combination of the lowest and higher order modes. An important design goal for many diode pumped solid state lasers is the generation of nearly diffraction limited TEM.sub.00 output at the highest possible efficiency and power. Because of the small TEM.sub.00 mode size of a confocal resonator, and its tendency to oscillate in a combination of the lowest and higher order modes, diode end pumping a confocal resonator has not been considered useful.
The effect of mode-matching is to maximize the coupling between the TEM.sub.00 mode and the excited volume in the crystal. In turn, the optical slope and the overall optical efficiencies of the laser are both maximized. In a classic mode-matched geometry, the ratio of the TEM.sub.00 mode diameter to the diameter of the pump beam in an Nd:YAG pumped laser is about 1.3 or greater. Because of the small TEM.sub.00 size, this ratio is more difficult to achieve with a confocal resonator.
The thermal lens in a laser crystal can be used in combination with mirror curvatures or conventional lenses to design a nearly confocal resonator. Certain crystals exhibit strong thermal lens characteristics, these laser crystals have other important properties that make them suitable candidates for laser resonators. For example, in comparison to Nd:YLF, the strong thermal lens material Nd:YVO.sub.4 has a high gain and a short upper state lifetime. These properties provide important adjustable parameters when designing a Q-switched laser with high pulse energy or high repetition rate, or a laser that is insensitive to optical feedback. Additionally, Nd:YVO.sub.4 has a high absorption coefficient at the diode pump wavelength of .about.809 nm, permitting efficient coupling of diode pump light into the Nd:YVO.sub.4 crystal.
Many laser crystals have strong thermal lenses, such as Nd:YAG and Nd:YVO.sub.4. With a strong thermal lens, the focussing power of the pump induced lens is at least comparable to that of the other optics in the laser resonator. A strong thermal lens significantly changes the size and divergence of a laser resonator eigenmode within the resonator.
With a weak thermal lens, the focussing power of the pump induced lens is substantially lower than that of the other optics in the laser resonator such as mirrors and typical lenses. The other optics in the laser resonator dictate the size and divergence of the resonator eigenmode.
It is clear that a thermal lens can be used to build a nearly confocal resonator. However, it has been thought that the large aberrations of strong thermal lens materials limit the efficiency of high power resonators. It has been generally believed that strong thermal lensing is a hindrance in the design and construction of an efficient laser with high TEM.sub.00 beam quality, high power and high efficiency. Therefore, successful use of strong thermal lens materials at higher pump powers has been limited.
Additionally, classical mode-matching has taught that TEM.sub.00 mode diameter to pump beam diameter ratios less than unity are of little interest since laser threshold is high and the achievable conversion efficiency is poor. Ratios of less than unity result in lower gain while those approaching and exceeding unity can result in higher loss due to aberration. It is more difficult to achieve a ratio greater than unity with a nearly confocal resonator since the TEM.sub.00 mode size is small. This feature goes against the teaching of conventional mode matching.
The use of diodes as pump sources for confocal resonators is desirable for reasons of cost, size and wall plug efficiency. Some strong thermal lens materials have certain properties that would make them useful for diode pumped lasers.
Another useful resonator type can be described as "in between" confocal and concentric. This can be called a confocal-to-concentric resonator. In Koechner, Solid State Laser Engineering, Vol. 3, p. 206, a resonator sensitivity analysis shows that a resonator with (g.sub.1)(g.sub.2)=0.5 features a mode size that is fairly insensitive to changes in the g parameter values. This means that small changes in thermal lens focal powers are relatively well tolerated in this type of laser. A number of laser resonators can be built with (g.sub.1)(g.sub.2)=0.5. This is represented by g values near point 25 in FIG. 1(a).
It would be desirable to have a compact, efficient low cost diode pumped laser that is sensitive to misalignment and has high pointing stability. It would also be desirable to provide a confocal-to-concentric resonator that has these properties, e.g. diode pumped, provides high output power, is highly efficient, and uses strong thermal lens materials. The laser may also have a TEM.sub.00 mode diameter in the laser crystal that is less than of the pump beam. It would also be useful to provide a Q-switched laser with similar characteristics, especially for applications that demand dynamic variations in the repetition rate of the Q-switched laser.